Liquid crystals can be divided into three classes based on molecular ordering: nematic, smectic, and cholesteric. Nematic liquid crystals are arranged with molecular directors parallel but not separated into layers. In smectic liquid crystals the molecules are arranged side by side in a series of layers. In the cholesteric phase, sometimes considered a subset of the nematic phase, the molecules are arranged in layers. Within each layer the molecular directors are parallel but not arranged in rows. The alignment of the molecular directors in each layer is slightly displaced from the adjacent layers, so that the molecular directors form a helical structure.
Nematic liquid crystals can provide variable retardation with response times on the order of milliseconds. They have been utilized in a number of applications including displays, spatial light modulators, switches and tunable filters. In twisted nematic cells the orientation of the molecular directors in the proximity of one substrate is at an angle to the orientation at the second substrate and the liquid crystal molecules in between are oriented to form a twisted structure with a twist angle between the orientations at the two substrates which can be electronically tuned. Twisted nematic cells are polarization waveguides which rotate polarization by the twist angle. Twisted nematic cells can provide a binary 90.degree. polarization change.
Chiral smectic liquid crystals (CSLCs) provide response times on the order of microseconds. When incorporated in a planar aligned geometry cell (smectic layers oriented perpendicular to the substrate walls), application of an electric field perpendicular to the cell walls reorients the molecular directors, providing electrooptical rotation. Analog CSLC materials, such as SmA* and distorted helix ferroelectrics (DHF) display an analog tilt of the cell optic axis in the plane of the cell walls. In a discrete, multi-state cell, for example using ferroelectric liquid crystal (FLC) SmC* or SmH* or antiferroelectric phases, application of an electric field above a certain threshold voltage results in switching the tilt of the CSLC molecules between discrete stable states. Recently the ferroelectric effect has been observed in achiral liquid crystals as well.
Cholesteric liquid crystals are characterized by the helical pitch, which is the distance through the film required for the molecular directors to trace a full 360.degree. cycle. Reflection of light is observed when the handedness of the incident polarization matches the helical sense of the cholesteric and when the wavelength, divided by the refractive index of the cholesteric, is approximately equal to the pitch. Thus cholesteric liquid crystals form cholesteric circular polarizers (CCPs), also known as cholesteric filters, which break unpolarized light into right- and left-handed circularly polarized components. The component with the same handedness as the cholesteric is reflected when the wavelength falls within the reflection band and transmitted at other wavelengths. The component with opposite handedness is transmitted at all wavelengths. In contrast to reflection from mirrors, the reflected light does not undergo a 180.degree. phase shift.
Liquid crystal molecules can be linked as a side chain to a polymer backbone to produce structures with the optical properties of liquid crystals and the glassy state of polymers. Cholesteric liquid crystal side chain polymers can be produced in a glassy state without changing the cholesteric optical properties. The polymer can also be coated on a variety of surfaces.
A second area of related art is color display technologies. For color display, the entire gamut of colors can be perceived using a high-speed three-color filter with a white input spectrum. The optimum filter would produce high throughput and high purity spectra for the red, green and blue primary color bands. Note that cyan, magenta and yellow are the perceived colors produced by removing red, green and blue, respectively, from white light.
Previous efforts to produce a liquid crystal tunable color filter have focused on polarization interference filters. However, because of the tradeoff between throughput and spectral purity, the Lyot structure is not optimum for implementing a three-color filter.
Color filters have been reported employing cholesteric circular polarizers (Buzak [1988] U.S. Pat. No. 4,726,663, and Kalmanash [1992], U.S. Pat. No. 5,082,354) or linear polarizing filters (Buzak [1987] U.S. Pat. No. 4,674,841, Buzak [1988] U.S. Pat. No. 4,770,500, and Kalmanash [1991] U.S. Pat. No. 4,991,941). These filters generally contain a means for modulating the polarization of light in combination with a linear or cholesteric polarizing filter which has a polarization dependent transmission spectrum. They generally employ nematic liquid crystal variable retarders in configurations wherein varying the magnitude of the retardance varies the output polarization. They do not employ smectic liquid crystal cells with rotatable orientation of the optic axis perpendicular to the direction of light propagation and indeed their device configurations are incompatible with rotatable orientation liquid crystal cells.